1. Field of the Invention
The invention pertains to a process for separating and crystallyzing anomers of of 2-deoxy-2,2-difluoro-D-ribofuranosyl-1-alkylsulfonate from an anomeric mixture.
2. State of the Art
M. Hofer, Chem. Ber., 93, 277 (1960) and Bhahacharya, J Org. Chem., 28, 428 (1963) described a process for preparing 1-chloro-2-deoxy-3,5-O-(di-p-toluoyl)-D-erythro-pentofuranose. Since the .alpha.-anomer was crystalline, it could be separated by filtration from the .beta.-anomer and obtained in the pure crystalline form. Thus, the crystalline chloride has become the predominant substrate for the synthesis of .beta.-nucleosides from 2-deoxyribofuranosyl derivatives.
The 2-deoxyribofuranosyl alkylsulfonate derivatives are unstable compounds. Therefore, there is little mention of them in the chemical literature. However, 2-deoxy-2,2-difluoro-D-ribofuranosyl-1-alkylsulfonate derivatives are stable compounds because of the presence of a gem-difluoro moiety at the 2-position. For instance, 2-deoxy-2,2-difluoro-D-ribofuranosyl-1-methanesulfonate are used in U.S. Pat. Nos. 4,526,988 and 4,965,374 in the preparation of pyrimidine and purine nucleosides. However, the 2-deoxy-2,2-difluoro-D-ribofuranosyl-1-methanesulfonates were anomeric mixtures and no attempt was made to separate the pure anomers. As a result the nucleoside syntheses were non-stereoselective.
To prepare 2'-deoxy-2',2'-difluororibofuranosyl nucleosides stereoselectively requires a high purity alpha or beta anomer of 2-deoxy-2,2-difluoro-D-ribofuranosyl alkylsulfonate derivatives. Since, these alkyl sulfonate derivatives are usually produced as an anomeric mixture there is a need for a process for separating the respective anomers.
An objective of the present invention is to provide a process for separating an anomeric mixture of alpha and beta 2-deoxy-2,2-difluoro-D-ribofuranosyl alkylsulfonates.
Another objective of the present invention is to provide a process for separating an anomeric mixture of alpha and beta 2-deoxy-2,2-difluoro-D-ribofuranosyl alkylsulfonates that produce the anomers in crystalline form.
Other objects and advantages of the present invention will become apparent from the following description of embodiments.